U.S. v. BDO Seidman, LLP, (7th Cir., July 2007), read opinion here.

 This case addresses the “common interest doctrine” which is often confused with the attorney/client privilege, but is an essential concept to grasp especially for multi-party litigation. It is, in effect, an exception to the rule that no attorney/client privilege attaches to communications between a client and an attorney in the presence of a third person. In effect, the common interest doctrine extends the attorney/client privilege to otherwise non-confidential communications in limited circumstances. The common interest doctrine will apply only “where the parties undertake a joint effort with respect to a common legal interest, and the doctrine is limited strictly to those communications made to further an ongoing enterprise. Moreover, communications need not be made in anticipation of litigation to fall within the common interest doctrine. (See footnote 6.) The court found that a memorandum that two joint venturers, BDO and Jenkens & Gilchrist, who consulted with their respective in house counsel, and also the outside counsel BDO  hired with respect to the legality of a proposed financial course of action they would recommend to their common clients– was within the scope of the common interest doctrine. Moreover, the common interest doctrine cannot be waived without the consent of all the parties and the voluntary disclosure by one member of a joint venture does not waive it with respect to the other member of the joint venture.

Delaware has addressed the topic, e.g.,  in American Legacy Foundation v. Lorillard Tobacco Co.,  2004 WL 2521289 (Del. Ch. 2004) (discussing attorney/client privilege waiver and common interest doctrine–also called "joint defense doctrine"). See generallyJenkins v. Bartlett, (7th Cir. U.S. Ct. App., April 23, 2007), read opinion here, which explained that :

"… there is an exception to the general rule that the presence of a third party will defeat a claim of privilege when that third party is present to assist the attorney in rendering legal services. (citation omitted)."

"… This exception applies both to agents of the attorney, such as paralegals, investigators, secretaries and members of the office staff responsible for transmitting messages between the attorney and client, and to outside experts engaged ‘to assist the attorney in providing legal services to the client’, such as accountants, interpreters…. Additionally, this exception reaches retained experts, other than those hired to testify, when the expert assists the attorney by transmitting or interpreting client communications to the attorney or formulating opinions for the lawyer based on the client’s communications. (citation omitted)."